(1) Field of the Invention
The present invention relates to a developer for electrostatic images. More particularly, the invention relates to a developer for elastrostatic images, which shows a good flowability at the developing step and a high offset resistance at the fixing step.
(2) Description of the Prior Art
A so-called magnetic brush method has been frequently adopted as one developing method for developing an electrostatic latent image formed by the electrostatic photography. As the developer for this magnetic brush developing method, there are ordinarily used a so-called two-component type developer comprising a blend of a powdery magnetic carrier such as iron and toner particles formed by dispersing a pigment in a binder medium, and a so-called one-component type developer composed of electrically conductive magnetic particles formed by dispersing a fine powder of a magnetic material such as triiron tetroxide, optionally with other pigment, into a binder resin and rendering the surfaces of the particles electrically conductive according to need. Toner particles or particles of a one-component type developer applied to an electrostatic image are transferred directly on a photosensitive sheet for electrostatic photography or optional on a transfer sheet, and then, they are fixed according to appropriate heating means.
From the viewpoints of the thermal efficiency, preventing of such accidents as fires and the operation speed at the fixing step, a method in which fixation is carried out under heat and pressure by using a roll is advantageous for heat fixation of developer particles. However, according to this fixation method, since an image of the developer particles is contacted with the surface of the fixing roller under heating and compression, there is caused a so-called offset phenomenon that parts of developer particles are transferred on the surface of the fixing roller, and the so transferred particles are re-transferred on a sheet on which the image is fixed, resulting in contamination of a photosensitive plate or transfer sheet.
For preventing occurrence of this disadvantage, there has been adopted means of covering the surface of the fixing roller with a material having an excellent parting property or a high stain resistance, such as a fluorine resin, and forming a film of an offset-preventing liquid such as a silicone oil on the surface of the fixing roller. However, when such means is adopted, the mechanism of the fixing apparatus is complicated and it is difficult to supply the offset-preventing liquid precisely in correspondence to the copying or printing operation.
As means for overcoming these defects, there has already been tried a process in which a substance capable of acting as a parting agent is incorporated in developer and at the fixing step using a roller, the substance is set free in the form of a liquid on the surfaces of the developer particles to thereby prevent occurrence of the offset phenomenon. For example, Japanese Patent Publication No. 3304/77 discloses a toner for developing electrostatic images comprising a coloring agent, a styrene resin and a low-molecular-weight polypropylene. This toner is advantageous in that by using a fixing roller to which an offset-preventing liquid is not supplied, the fixing operation can be performed at a high efficiency in good conditions while preventing occurrence of the offset phenomenon. However, this toner is still insufficient or unsatisfactory in properties required for development.
More specifically, in the toner of this type, it is indispensable that a parting substance should be liquefied and set free on the surfaces of toner particles at the fixing step. Therefore, it is necessary that a relatively large amount of the parting substance should be incorporated in toner particles. If the parting substance is incorporated in a large quantity in toner particles, the flowability and charge characteristics of the toner particles are drastically reduced. Most of parting substances used for this purpose are soft in the normal state, and toner particles containing such parting agent in the surface portion tend to agglomerate and the toner has not a sufficient flowability for coating the toner uniformly on a magnetic brush-forming developing roller (sleeve). For example, agglomerates or masses of the toner particles are formed on the surface of the sleeve, and falling of such agglomerates or masses causes contamination of the background. Moreover, unevenness of adhesion of the toner particles to the surface of the sleeve, blurring is readily caused in the obtained image.
Electrostatic images formed according to a commercial electrostatic photocopying or printing process are generally negatively charged, and therefore, toner particles having a positive charge polarity are customarily used. Most of parting agents incorporated in relatively large quantities in toner particles have bad influences on the above charge characteristics of the toner particles. For example, we have confirmed that the above-mentioned polypropylene has a function of reducing the property of being positively charged in the toner particles.
This disadvantage by incorporation of a relatively large amount of the parting substance in toner particles becomes conspicuous in an electrostatic photocopying process of the type where a toner image formed on an electrostatic photographic photosensitive plate is transferred onto a transfer sheet and this photosensitive plate is used repeatedly. More specifically, the parting substance present on the surfaces of the toner particles is transferred onto the surface of the photosensitive plate and gradually accumulated thereon, and finally, an electrically insulating layer is formed on the surface of the photosensitive plate. Therefore, when such toner is used, even if the photosensitive plate is used repeatedly only a limited number of times, fogging of obtained prints or electric breakdown of the photosensitive plate is caused by residual charges, and the life of the photosensitive plate is drastically shortened.